Liquid crystal display devices are attracting attention as flat panel displays which achieve lightness and low power consumption. Among them, an active matrix type liquid crystal display device arranged with a switching element such as a thin film transistor in each pixel can obtain a high precision display image with no crosstalk and therefore is being utilized for each type of display beginning with screens for mobile terminals. A semiconductor thin film such as amorphous silicon or polysilicon is used as an active layer of a thin film transistor.
In order to achieve a small scale and narrow frame active matrix type display device, a display device is manufactured in which a drive circuit part for driving each pixel is integrally formed above the same substrate as a pixel part.
An example of an active matrix type liquid crystal display device is disclosed in Japanese Laid Open Patent Publication No. H02-304532. A thin film transistor of a liquid crystal display device in this example includes a dual gate structure in which a gate electrode is arranged both above and below an amorphous silicon layer which forms a channel. An operation pulse signal of a frame cycle is supplied to a first gate electrode located on the lower side, and an inverted pulse voltage is supplied in a frame cycle to a second gate electrode located on the upper side. According to the description in Japanese Laid Open Patent Publication No. H02-304532, by adopting this structure, it is possible to remove the effects of a small voltage variation on a channel upper part of a thin film transistor due to polarity inversion which is essential for liquid crystal display, and thereby it is possible to realize a uniform and high quality display across the entire screen.